The present invention relates to a centrifugal compressor and a diffuser used therein, and more particularly, to a centrifugal compressor and a centrifugal blower to handle comparatively low flow rate gas, and a diffuser used therein.
In a high-pressure centrifugal compressor to handle high pressure gas, phenomena disturbing safe operation of the compressor such as noise, damage to an impeller, and vibration of shafting easily occur in comparison with a general centrifugal compressor. One of the phenomena is rotating stall.
The rotating stall occurs mainly in a comparatively-low specific speed impeller stage. It is considered as the mechanism of the rotating stall that the rotating stall occurs due to a reverse flow which occurs in a flow in the diffuser. The flow in the diffuser is a deceleration flow, and separation of the flow from a wall surface easily occurs in accordance with inverse pressure gradient. This phenomenon easily occurs on the downstream side in accordance with increase in a ratio of a flow channel height of the diffuser to an outlet radius of an impeller. It is considered that the separation of the flow gradually increases, which leads to the rotating stall.
In the centrifugal compressor in which the rotating stall may occur, a technique using a vaned diffuser is disclosed in International application WO97/33092. According to this technique, a vaned diffuser with a constant flow-channel height and a low solidity (a low chord-pitch ratio) is provided on the downstream side of the impeller, and on its downstream side, a vaneless diffuser in which the flow channel height decreases in a flow direction is provided. In this structure, the efficiency of the compressor is improved while the rotating stall is prevented.
However, in the technique using the vaned diffuser as disclosed in the above-described International application WO97/33092, usage in a high-pressure centrifugal compressor is not sufficiently considered.
That is, in some cases, a comparatively-low specific speed (specific speed: about 200 and/or lower) impeller using wedge-shaped thick impeller blades is employed in a high-pressure comparatively-low specific speed centrifugal compressor. In a comparatively-low specific speed region, the performance of the thick blade impeller is greater than that of a general thin blade impeller. However, in the thick blade impeller, as the impeller height is higher in comparison with a thin blade impeller with the same flow rate, a radial component of the speed at a diffuser inlet is small. Accordingly, a flow angle is small. Further, by a wake flow from a trailing edge of the thick blade impeller, a flow at a small flow angle locally occurs in circumferential speed distribution at the impeller outlet. Accordingly, a reverse flow in the diffuser easily occurs in comparison with a thin blade impeller stage with the same flow rate. In this manner, in the conventional high pressure centrifugal compressor, prevention of rotating stall is not considered.